This invention relates to a silicoaluminophosphate molecular sieve (SAPO-11) with AEL-type structure, its synthetic method, and a catalyst containing the same, especially a catalyst for hydrocarbon hydroisomerization.
Aluminophosphate molecular sieves are molecular sieves of a new generation developed by UCC of the United States of America in early 1980""s (U.S. Pat. No. 4,310,440) following the aluminosilicate molecular sieves. The typical character of this class of molecular sieve is that their framework is constructed by alternative connection of phosphorus-oxygen tetrahedrons and aluminum-oxygen tetrahedrons, and since the framework of the molecular sieves appears electrically neutralized, they have no capability for carrying out cation exchange and catalytic reaction.
The aluminophosphate (AlPO4- 11) molecular sieve with AEL-type structure is a member of the aluminophosphate molecular sieve family, which belongs to the orthorhombic crystal system with a space group of Ima2. Its crystal unit cell parameters are a=18.7 xc3x85, b=13.4 xc3x85, and c=8.4 xc3x85, and the one dimensional pore size of its 10-member ring is 3.9xc3x976.3 xc3x85. Its typical X-ray diffraction data are listed in Table 1. After removing amine by calcination, it still belongs to the orthorhombic system, but its symmetry is changed and the space group turns to Pna21, with the crystal unit cell parameters of a=18.1 xc3x85A, b=13.8 xc3x85, and c=8.1 xc3x85. Its X-ray diffraction pattern is apparently different from that before calcination, and the data of a typical X-ray diffraction pattern is listed in Table 2.
Silicoaluminophosphate molecular sieves, i.e. SAPO molecular sieve series are formed when silicon is incorporated into the framework of aluminophosphate molecular sieves (UCC of USA, U.S. Pat. No. 4,440,871). Their framework is constructed with phosphorus-oxygen tetrahedrons, aluminum-oxygen tetrahedrons, and silicon-oxygen tetrahedrons, and since their framework carries negative charge, they have non-framework cations in balance and the capability of cation exchange. If the non-framework cations are H+, they have acidic catalytic capability since they have acidic centers.
The aluminophosphate molecular sieve with AEL structure containing silicon (SAPO-11) has the same structure and XRD pattern as that containing no silicon (AlPO4-11), but after removing amine by calcination, the structure of a molecular sieve has different states. According to the results reported in U.S. Pat. No. 4,440,871, the typical data of the X-ray diffraction pattern of a synthesized silicoaluminophosphate molecular sieve with AEL structure are the same as listed in Table 1. After removing amine by calcination, the data of the X-ray diffraction patterns are different depending on the raw materials adopted. When the molecular sieve is synthesized using phosphoric acid as a phosphorus source, aluminum isopropoxide as an aluminum source, fuming silica gel as a silicon source, and di-n-propylamine as a template, its data of X-ray diffraction pattern after removing amine by calcination is partly changed with the appearance of the diffraction peaks at 2xcex8=12.8, 16.1, and 21.9xc2x0, etc, and the newly appearing peaks are substantially the same as the data of the X-ray diffraction pattern of the aluminophosphate having AEL structure and containing no silicon (AlPO4-11) after removing amine by calcination, showing that the crystal structure of the molecular sieve synthesized by this method is partly changed after removing amine by calcination. When the molecular sieve is synthesized by using phosphoric acid, aluminum isopropoxide, silica sol and di-n-propylamine as the raw materials, the data of the X-ray diffraction pattern are remarkably changed with the appearance of the diffraction peaks at 2xcex8=9.85, 12.8, 16.1, and 21.95xc2x0, and the thorough disappearance of the peaks at 2xcex8=9.4, 13.1, 15.65, and 21.1xc2x0 at the same time. These data are the same as the data of the X-ray diffraction pattern of the aluminophosphate molecular sieve containing no silicon (AlPO4-11) after removing amine by calcination. These results suggest that the structure of the molecular sieve with AEL structure after calcination is different depending on its composition and synthetic method.
U.S. Pat. Nos. 4,943,424 and 5,208,005 also disclose a molecular sieve with AEL structure (SM-3) and its synthetic method. The data of the X-ray diffraction pattern of said molecular sieve are substantially the same as those of the molecular sieve disclosed in U.S. Pat. No. 4,440,871. After removing amine by calcination, however, its X-ray diffraction data are completely the same as those of the molecular sieve with AEL structure containing no silicon after removing amine by calcination, indicating that the structure of the molecular sieve is also changed after removing amine by calcination. The other feature of the molecular sieve emphasized by the two patents is the enriched silicon on the surface of the molecular sieves derived by their synthetic method.
Regarding the method for synthesizing the aluminophosphate and silicoaluminophosphate molecular sieves with AEL structure, the synthetic method described in U.S. Pat. No. 4,310,440 comprises: taking phosphoric acid as a phosphorus source, hydrated alumina (pseudo-boehmite) as an aluminum source, di-n-propylamine or di-isopropylamine, ethylbutylamine, di-n-butylamine, di-n-pentylamine, as an organic template, adding hydrated alumina into the aqueous solution of phosphoric acid in a ratio of 1.0R: P2O5: Al2O3: 40H2O, stirring to uniformity, adding the organic template after stirring to uniformity, sealing the mixture into a stainless steel autoclave lined with Teflon after stirring to uniformity, crystallizing at 200xc2x0 C. for 24-48 hours, and then filtering, washing, and drying, to yield the molecular sieve product.
In the method provided in U.S. Pat. No. 4,440,871 for synthesizing a silicoaluminophosphate molecular sieve with AEL structure, the phosphorus source used is phosphoric acid, the aluminum source is aluminum isopropoxide or hydrated alumina, the silicon source is fuming silica gel or silica sol, and the organic template is di-n-propylamine or di-isopropylamine. When aluminum isopropoxide is used as the aluminum source, phosphoric acid was first added into the mixture of aluminum isopropoxide and water, and after stirring to uniformity, fuming silica gel is added. Then di-n-propylamine is added after stirring and the stirring is continued until the mixture becomes uniform. The mixture is sealed into a stainless steel autoclave lined with Teflon, and crystallized at 150-200C. to obtain the molecular sieves. When hydrated alumina (pseudo-boehmite) is used as the aluminum source, the hydrated alumina was added into the aqueous solution of phosphoric acid, and after stirring to uniformity, the mixture of fuming silica gel and tetrabutylammonium hydroxide is added. The mixture is stirred to uniformity, and the template di-n-propylamine is added. Then crystallization is carried out after stirring to uniformity to obtain the molecular sieve product. When aluminum isopropoxide is used as the aluminum source, and silica sol is used as the silicon source, the structure of the obtained molecular sieve is thoroughly changed after removing amine by calcination. It is worthy to note that, in the method provided in the aforesaid patents, the influence of the gelation temperature has not been mentioned.
In the method provided by U.S. Pat. Nos. 4,943,424 and 5,208,005 for synthesizing a silicoaluminophosphate molecular sieve with AEL structure, phosphoric acid, aluminum isopropoxide, fuming silica gel and di-n-propylamine are used as the raw materials. Under the ice bath condition, aluminum isopropoxide is added into the aqueous solution of phosphoric acid, and fuming silica gel or a mixture of fuming silica gel and water is added after mixing to uniformity. Then di-n-propylamine is added, and after mixing or grinding, the mixture was charged in a stainless steel vessel for crystallization, and the molecular sieve product is obtained. This method emphasis that the pH value after gelation should be adjusted to 6.0-8.0, and the optimum crystallization temperature is in the range of 170-240xc2x0 C. The crystal structure of the product obtained is changed after removing amine by calcination.
The technology of the shape selective isomerization of hydrocarbon oil is well known. Generally, this technology is applied to treat wax oil for reducing the content of normal paraffins. In order to improve the performance of the oil products, normal paraffins, especially long chain normal paraffins presented in the oil products should be removed as much as possible. For example, the octane number of the gasoline fraction can be boosted by removing the straight chain paraffins through shape selective cracking or converting them into branched paraffins by isomerization. For diesel oil or lubricant oil, their freezing point or pouring point can be lowered by removing the straight chain paraffins to improve their low temperature performance. Another key problem is the yield of the target product in the above reaction process. Since the reactant can be converted to lower hydrocarbons with smaller molecules by cracking, the yield of the target product may be lowered. Therefore, the isomerization reaction should be enhanced as much as possible, and the cracking reaction should be reduced as much as possible at the same time.
In view of the thermodynamic point, cracking reaction needs relatively strong acidic centers in the catalyst and relatively high reaction temperatures, and relatively weak acidic centers in the catalyst and relatively low reaction temperature are beneficial to the isomerization reaction. However, in order to increase the reaction activity and overcome the shortcomings of low reaction efficiency caused by low activity of the catalyst with weak acid centers and low reaction temperature, it is needed to load active metal components with hydrogenation and dehydrogenation functions for preparing bifunctional catalysts.
Isomerization catalysts with silicoaluminophosphate molecular sieves as an acidic active component are described in U.S. Pat. Nos. 4,710,485 and 5,087,347.
U.S. Pat. No. 4,710,485 discloses a technology using a silicoaluminophosphate molecular sieve as an isomerization catalyst. The silicoaluminophosphate molecular sieves are selected from mesopore molecular sieves such as SAPO-11or SAPO-41. SAPO-11 and SAPO-41 molecular sieves are synthesized according the method disclosed in U.S. Pat. No. 4,440,871. Their properties are also the same as those of the molecular sieves disclosed in U.S. Pat. No. 4,440,871, especially the SAPO-11 molecular sieve. Its characteristic peaks of the X-ray diffraction pattern are the same as those of the molecular sieve disclosed in U.S. Pat. No. 4,440,871, which is changed to a certain extent after calcination at high temperature, i.e., the characteristic of the SAPO-11 molecular sieve used in the technology disclosed in U.S. Pat. No. 4,710,485 which uses silicoaluminophosphate molecular sieves as the isomerization catalyst is that the structure of the molecular sieves is partly or completely changed after high temperature calcination.
U.S. Pat. No.5,087,347 discloses a technology using a silicoaluminophosphate molecular sieves as the isomerization catalyst, in which the silicoaluminophosphate molecular sieve is selected from mesopore molecular sieve SM-3. Molecular sieve SM-3 has identical X-ray diffraction pattern with SAPO-11 molecular sieve disclosed in U.S. Pat. No. 4,440,871, but the surface composition of the SM-3 molecular sieve is different from that of the SAPO-11 molecular sieve, in particular, the surface is silicon enriched. When the isomerization catalyst with the SM-3 molecular as an acidic component is used in the n-octane conversion reaction, its activity is enhanced. When the SM-3 molecular sieve is calcined at a high temperature, its XRD spectra have the features of the characteristic peaks shown in Table 2, that is, its structure changes completely.
The crystal structures of all the molecular sieves reported in the prior art are changed after removing template by calcination, and therefore it is taken for granted that the crystal structure of the SAPS-11 molecular sieve should be changed after removing template by calcination. But the inventors of this invention have found that by controlling certain synthetic conditions, the structure of the SAPO-11 molecular sieve can be stabilized, that is, after removing template by calcination, the structure of the molecular sieve remains substantially unchanged. Further more, when the SAPO-11 molecular sieve having a stable crystal structure is used as an acidic component in the catalyst for paraffin hydroisomerization, the isomerization selectivity and isomerization product yield can be significantly increased.
Based on the above description, the objective of this invention is to provide a silicoaluminophosphate molecular sieve with AEL structure (SAPO-11), the data of the X-ray diffraction pattern of which is substantially unchanged after removing template by calcination in comparison with those before removing the template by calcination. This means that the molecular sieve has superior structure stability.
Another objective of this invention is to provide a method for preparing said molecular sieve.
A further objective of this invention is to provide a catalyst containing said SAPO-11 molecular sieve and noble metals. When this catalyst is used in hydrocarbon hydroisomerization, both isomerization selectivity and isomerization product yield are significantly increased.
A SAPO-11 silicoaluminophosphate molecular sieve, its preparation method, and a catalyst containing the same are provided in this invention. The X-ray diffraction data of said molecular sieve before removing the template by calcination are as shown in Table 1, its molar composition after removing the template by calcination expressed in anhydrous oxides is Al2O3:yP2O5:zSiO2, in which y has a value of 0.60-1.20, and z has a value of 0.05-1.3, characterized in that after removing the template by calcination, said molecular sieve has the main X-ray diffraction data as listed in Table 3, and the crystal structures of the molecular sieve before and after removing the template by calcination are substantially the same. Said catalyst is composed of 10-85% by weight of said SAPO-11 molecular sieve, 0.05-1.5% by weight of Pd or Pt, and alumina in balance. In comparison with similar catalysts in the prior art, both isomerization selectivity and product yield are significantly increased when the catalyst of this invention is applied in hydrocarbon hydroisomerization reaction.
The major X-ray diffraction peaks of the silicoaluminophosphate molecular sieve with AEL structure (SAPO-11) provided in this invention before removing the template by calcination are as listed in Table 1. Its molar composition after removing the template by calcination expressed in anhydrous oxides is Al2O3:yP2O5:zSiO2, in which y has a value of 0.60-1.20, z has a value of 0.05-1.3, characterized in that its X-ray diffraction data after removing the template by calcination are as listed in Table 3, in which the conditions of said calcination are the conventional conditions used in the prior art for removing the template in this class of molecular sieves. The molar composition of the silicoaluminophosphate molecular sieve with AEL structure (SAPO-11) provided in this invention before removing the template by calcination expressed in anhydrous oxides is xR:Al2O3:yP2O5:zSiO2, wherein R is the organic template presented in the channels of the molecular sieve, and may be an organic template conventionally used in the prior art, with di-n-propylamine or di-isopropylamine or their mixture being preferred; x has a value of 0.01-0.35, preferably 0.03-0.25; y has a value of 0.60-1.20, preferably 0.75-1.05; and has a value of 0.05-1.3, preferably 0.1-1.1.
The positions of the XRD peaks of the silicoaluminophosphate molecular sieve provided in this invention before calcination are the same as those of the aluminophosphate molecular sieve with AEL structure (its main diffraction peaks are as listed in Table 1), indicating that it has AEL crystal structure. From the data of Table 3, it can be seen that the XRD peaks of the molecular sieve after calcination have substantially the same positions as those of the molecular sieve before calcination, though the intensities of the XRD peaks are slightly different. This indicates that the crystal structure of the molecular sieve provided in this invention is very stable, its crystal structures are substantially the same before and after calcination.
The method provided in this invention for synthesizing silicoaluminophosphate molecular sieves with AEL structure comprises: mixing an aluminum source, a silicon source, a phosphorus source, and an organic template to make a gelatinous reaction mixture with a molar composition of aR: Al2O3: bP2O5: cSiO2: dH2O, then crystallizing the mixture by steam treating, and filtering, washing, drying, and calcining the crystallized product, characterized in that said gelation temperature is in a range of 25-60xc2x0 C., preferably 28-42xc2x0 C., more preferably 30-40xc2x0C., said crystallization conditions are a temperature range of 140-190xc2x0 C., preferably 150-180xc2x0 C., more preferably 160-175xc2x0 C., a autogenous pressure, and a duration of 4-60 hours, preferably 10-40 hours; a has a value of 0.2-2.0, preferably 0.3-1.5, more preferably 0.5-1.0; b has a val of 0.6-1.2, preferably 0.8-1.1; c has a value of 0.1-1.5, preferable 0.3-1.2; d has a value of 15-50, preferably 20-40, more preferably 25-35.
Said sources of aluminum, silicon and phosphorus, and organic template in the method provided in this invention are the corresponding raw materials typically used in the prior art. Said aluminum source includes aluminum hydroxide, hydrated alumina, aluminum isopropoxide or aluminum phosphate; said silicon source includes solid silica gel or silica sol; said phosphorus source includes phosphoric acid or aluminum phosphate; and said organic template includes di-n-propylamine, di-isopropylamine or their mixture.
In the method provided by this invention, said calcination conditions are the conditions typically used in the prior art, in which the preferred conditions are at 500-650xc2x0 C. for 2-10 hours.
The key of the method provided in this invention is that the gelation temperature of the raw materials is controlled at an adequate range slightly higher than the room temperature, and at the same time, the crystallization temperature is controlled at a relatively low temperature range. If the gelation temperature exceeds the range provided in this invention, or the crystallization temperature is higher than 200xc2x0 C., the structure stabilized SAPO-11 of this invention can not be obtained.
The molecular sieve provided in this invention can be used as a component of the catalysts for hydrocarbon isomerization, catalytic dewaxing, freezing point depression of diesel oil or lubricant oil, etc, especially can be used as a catalyst for hydrocarbon hydroisomerization after supporting noble metals. In order to make the molecular sieve into a metal containing bifunctional catalyst, the molecular sieve can be firstly calcined to remove template, and then impregnated with metal components, or firstly impregnated with metal components, and then calcined to remove template. The noble metal can be Pt, Pd, or the mixture thereof.
The silicoaluminophosphate molecular sieve with AEL structure provided in this invention or obtained by the method provided in this invention has superior structural stability, that is, after removing the template by calcination, its XRD pattern data are substantially unchanged in comparison with those before removing the template by calcination. In comparison with the catalysts containing molecular sieves obtained with the prior art, when this molecular sieve is impregnated with palladium or platinum components and used for hydrocarbon hydroisomerization reaction, the isomerization selectivity and product yield are significantly increased.
The hydrocarbon hydroisomerization catalyst provided by this invention is comprised of 10-85 wt. % of silicoaluminophosphate molecular sieve with AEL structure (SAPO-11), 0.05-1.5 wt. % of Pd or Pt, and alumina in balance, wherein the molar composition of said SAPO-11 molecular sieve expressed in anhydrous oxides is Al2O3: (0.60-1.20) P2O5: (0.05-1.3) SiO2, and characterized in that the main X-ray diffraction pattern data of said SAPO-11 molecular after removing the template by calcination are as listed in Table 3. Said calcination conditions are the conventional conditions in the prior art for removing the organic template in the same class of molecular sieves.
The preferred catalyst provided in this invention is composed of 20-80 wt. % of said SAPO-11 molecular sieve, 0.1-1.2 wt. % Pd or Pt, and a balanced amount of alumina.
The method provided in this invention for preparing the catalyst is the conventional impregnation method in the prior art, which comprises: mixing, kneading, and molding said SAPO-11 molecular sieve, a precursor of alumina, deionized water, and nitric acid; drying and calcining the formed product; supporting a compound of Pd or Pt or their mixture by impregnation; drying and calcining the impregnated support to yield the catalyst of this invention, wherein said precursor of alumina may be a precursor commonly used in the prior art, which converts to xcex3-Al2O3 after calcination, no other limitation being placed on by this invention. The precursor can be one or several compounds selected from the group consisting of amorphous aluminum hydroxide, pseudo-beohmite, alumina tri-hydrate and bayerite, wherein pseudo-beolimite is preferred. Said alumina in this invention is the alumina obtained by calcining the above precursors at 400-700xc2x0 C. for 1-5 hours.
Said SAPO-11 molecular sieve can be calcined to remove the organic template either before, or after said catalyst is molded by extruding. No matter the calcination proceeds before or after extruding, the molecular sieves in the catalysts of this invention can all keep the stable crystal structure. Said calcination conditions for removing the template are those typically used in the prior art, in which the preferred conditions are keeping consistent at 500-650xc2x0 C. for 2-10 hours.
The catalyst provided in this invention can be used for isomerization, catalytic dewaxing of hydrocarbon, freezing point hydro-depression of diesel oil or lubricant oil, etc., especially for hydroisomerization of the lubricant oil with a boiling range of 350-580xc2x0 C. and the diesel oil with a boiling range of 160-400xc2x0 C. In order to make the molecular sieve into a metal containing to bifunctional catalyst, the molecular sieve can be firstly calcined to remove template, and then impregnated with a metal component, or firstly impregnated with a metal component, and then calcined to remove the template.
Since the silicoaluminophosphate molecular sieve with AEL structure provided by this invention has superior structure stability, which is shown by its substantially unchanged data of the XRD pattern after removing the template by calcination in comparison with those before removing the-template by calcination. Compared with the catalysts in the prior art, the catalyst displays higher isomerization selectivity and isomerization product yield when used in hydroisomerization of hydrocarbon.
This invention will further be illustrated by the following examples. The compositions of the molecular sieves in the examples and comparative examples are detected by X-ray fluorescence spectroscopy.